DFT-SOFDM is an OFDM-like single carrier modulation technique that is used in the EUTRA uplink (36.211 v8.2.0). DFT-SOFDM has significantly better power de-rating, which is also known as cubic metric or peak to average power ratio (PAPR) properties than OFDM, enabling better data rates near the cell edge and/or improved battery life in subscriber stations or user equipment (UE).
Unfortunately, direct conversion transmitters and receivers introduce distortion on the DC sub-carrier. On the uplink, the distortion includes unsuppressed carrier feed-through from all active user equipment in the uplink. The DC sub-carrier may be used for DFT-SOFDM transmissions. Allocations spanning the DC sub-carrier but not transmitting or skipping over the DC sub-carrier would suffer from increased CM (˜1.7 dB for pi/2 BPSK, 0.7 dB for QPSK, and 0.5 dB for 16QAM for DC distortion power equal to the average sub-carrier power negating the low CM benefit of DFT-SOFDM. At the transmitter, both error vector magnitude (EVM) and the cubic metric (CM)/PAPR worsen with increased levels of DC distortion. In 3GPP EUTRA 36.211 v8.2.0, the transmitted signal is frequency shifted by ½ the sub-carrier spacing, prior to transmission so that the impact of the DC distortion due to local oscillator leakage can be distributed over sub-carriers adjacent to the DC sub-carrier.
Since DFT-SOFDM is a weighted sum of multiple data symbols (in contrast to conventional OFDM), degradation to the DC sub-carrier degrades receiver performance compared to an ideal DFT-SOFDM receiver with no DC distortion. Simply discarding (zeroing out) the DC sub-carrier at the receiver can cause 0.9 dB of degradation for 1 resource block (12 sub-carrier) allocations with 16QAM modulation. On the other hand, not discarding the DC subcarier results in significant degradation for high DC distortion values. Therefore, there is a need for improved DC sub-carrier handling in the receiver for DFT-SOFDM uplink.
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